Current mode circuitry has significant advantages over voltage mode circuitry. The maximum current level for an electronic circuit is not fundamentally limited by the circuit's power supply voltage, but instead is limited only by power dissipation and component size. This advantage is significant for sub-micron processes having low supply voltage limits. Also, current signals are less subject to negative influences, such as ground and power supply noise, debiasing, and signal line impedance. Due to physical limitations of packaging and interconnection technology, these negative influences are further increased at higher operating frequencies, thus making current mode circuitry preferred for many high speed operations.
In interfacing with off-chip circuitry, standard integrated circuit ("IC") packaging uses metal lead frames to which the chip is attached and the leads are bonded. Parasitic inductances of these lead frames create large voltage differences from charging or impulsive currents, thereby negatively impacting high speed IC interfaces with voltage mode signals. Comparatively, such problems are substantially avoided for IC interfaces with current input signals from a high impedance source.
Moreover, several current mode functions do not require linear settling active gain components such as operational amplifiers, and many current mode operations incorporate low gain open loop signal processing techniques to avoid significant node voltage changes and bandwidth limitations of high gain feedback loops. Additionally, transconductance-C, MOSFET-C and other integrated continuous time filtering technologies may be configured for current output, offering favorable performance for signal bandwidths in the megahertz ("MHz") frequency range.
Current input analog to digital ("A/D") converters may be directly interfaced to switched current filters, current conveyors, and other current mode circuitry, thereby providing very high speed, supply noise rejection, and relief from package inductance problems. Moreover, high speed current input A/D converters complement the functions of commercially available high speed current steering digital to analog ("D/A") converters.
Previous approaches to current input A/D conversion typically fail to incorporate a parallel input or flash type architecture, in which a single comparator stage converts all digital bits in parallel. Instead, these previous approaches commonly use successive approximation techniques, in which a series of D/A current levels are successively compared against a current input to achieve recirculated bit operations. For example, a 10- bit resolution converter requires 10 successive current signal comparisons, effectively limiting the speed of such a successive approximation technique to a tenth of the speed possible with a parallel input architecture. For a successive approximation technique using recirculated bit operations, a typical 10-bit resolution converter provides 25 KHz sample rates. Even when a successive approximation technique is implemented to achieve cascaded or pipelined bit operations at the shortcoming of latency and additional hardware, a typical 6-bit resolution converter provides 200 Khz sample rates. Nevertheless, higher speeds are desirable; for example, integrated circuits in video applications may switch between pixels at frequencies from 10 MHz to greater than 100 MHz.
Some hybrid A/D conversion techniques combine current mode techniques with voltage mode techniques to achieve 10-bit resolution with sample rates from 20 MHz to 75 MHz and possibly higher. Nevertheless, many hybrid techniques are undesirably complex, because they require conversions between current and voltage modes during the A/D conversion process. Consequently, a complete current mode technique is desirable, such that conversions between current and voltage modes are not required.
Thus, a need has arisen for a method and circuitry for current input analog to digital conversion, such that potential advantages of current mode circuitry may be achieved. Moreover, it is desirable to provide higher sample rates for current input A/D conversion relative to typical previous approaches, and to use parallel input architecture for current input A/D conversion. Finally, a need has arisen for analog to digital conversion wherein conversions between current and voltage modes are not required.